1. Field of the Invention
This invention relates to a method for detecting pit patterns on a disk-shaped recording medium of a sampled servo system.
2. Description of the Prior Art
For a conventional disk-shaped recording medium, including an optical disk or a magneto-optical disk, such as, for example, a 3.5-inch magneto-optical disk, there has been proposed a recording format as shown for example in FIG. 1. For this recording format, a so-called 4-11 modulation system is employed, according to which one byte or 8-bits of data are recorded by an 11 channel bit pit pattern.
Referring to FIG. 1a, there is shown a 22 channel bit sector mark, corresponding to 2-bytes of data, after a 22 channel pit servo byte corresponding similarly to 2 bytes of data, at the leading position of one sector. This is a pit pattern appearing at the leading portion of each sector, as will be described in more detail later. Referring to FIG. 1a, a servo byte comprises gray code pits, tracking pits P.sub.T1, P.sub.T2, "wobbled" with respect to a track center T.sub.kc, and a clock pit P.sub.CK, while a sector mark comprises sector mark pits P.sub.SM located 14 channel pits after the tracking pit P.sub.T2 of the servo byte.
As shown in FIG. 1b, the 2-byte servo byte and the 2-byte sector mark comprise a segment along with track and sector numbers, such as most significant bits and least significant bits. The sector mark, track numbers and the sector numbers make up 8 bytes. Meanwhile, as shown in FIG. 1c, each sector comprises 76 segments, of which the leading three segments are headers 1 to 3 and the remaining 73 segments are data. The segment shown in FIG. 1b is located at the foremost segment of the sector, that is at header 1, as shown in FIG. 1c.
This sector is organized as shown in FIG. 2, in which the header region is formed by six rows. Segment 1 of the header region comprises the 2-byte servo byte and a 2 rows.times.4 byte identification label ID including 2-byte sector mark. Segment 2 comprises the servo byte and the preamble/ALPC. Segment 3 comprises the servo byte and the preamble. The data region comprises servo bytes and 2 rows.times.3 bytes of data in 130 rows. Segment 68 includes 2 bytes of control data and cyclic redundancy check (CRC) data. The error correcting code (ECC) region comprises servo bytes and 2 rows.times.4 bytes of ECC data in 16 rows.
The two pits of the gray code of the servo bytes are recorded at a period of 16 tracks, as shown for example in FIG. 3.
In a 3.5 inch magneto-optical disk, on which data are recorded in accordance with the above mentioned 4-11 modulation system, the bit data obtained upon reading the sector mark pits P.sub.SM are "10010010001001". To distinguish the format pits from data pits recorded on the disk, the data pits are recorded using a pattern not usually found in the data region.
The circuit to detecting the pit pattern of the sector mark shown in FIG. 4 is employed. Referring to FIG. 4 reproduced signals read, for example, from a magneto-optical disk as the disk-shaped recording medium are input to an input terminal 101. The reproduced signal is differentiated by a differentiating circuit 102 and processed in a comparator 103 so as to be converted at a predetermined threshold level into a rectangular wave signal which is transmitted to a shift register 104. A sync signal and a reference clock signal are transmitted to the shift register 104 via input terminals 105 and 109, respectively, so that a signal Z produced by synchronization by the sync signal and shifting from the rectangular wave signal on the basis of the reference clock is transmitted to a comparator 106. A bit data pattern W of "10010010001001" is supplied to the comparator 106 via input terminal 107. A sector mark detection signal is output via output terminal 108 when Z=W="10010010001001".
Referring to FIG. 1a, the servo byte and the sector mark are adjacent in the above format, and a pattern consisting of 14 bit intervals or 13 "0" bits between the servo bytes and the sector marks, that is, a unique pattern, is formed. By detecting the unique pattern, the servo byte may be detected once for each sector. The unique pattern can be detected by a circuit similar to that of FIG. 4 when the data to be compared in the comparator 106 is the bit data pattern "100000000000001".
In general, when the data recorded on a disk is modulated, the capacity that may be recorded on a disk is less than in the case of unmodulated data. However, without modulation there is a risk that the pits of the same pattern as the sector mark may be found in the data region. In such case, the sector mark pattern cannot be detected accurately.
Although there is one servo byte per segment as shown in FIG. 1c, the unique pattern between the servo byte and the sector mark which is not found in the 4-11 modulation system data region appears only once in each sector. Therefore, if the phase locked loop (PLL) which performs capturing by detecting the clock pit P.sub.CK of the servo byte during disk reproduction, that is, reading should be disengaged, re-capturing is not feasible until after passage of one or more sectors.